The present invention concerns voltage regulators, and more particularly, a voltage regulator wherein the efficiency of the regulator is improved.
The present voltage regulator is useful in a direct broadcast satellite receiver system which includes an outdoor microwave antenna which can be aimed at a satellite to receive a signal from the satellite. The signal received from the satellite is amplified by a "low noise block converter" (LNB) mounted in very close proximity to or on the antenna.
The output signal from the LNB is carried to an indoor receiver by a coaxial cable. In order to supply power from the indoor receiver to the LNB, as well as to control the polarization of the LNB, a DC voltage is multiplexed onto the center conductor of the coaxial cable. The circuits in the LNB are designed so that they will function with either a lower power supply voltage or a higher power supply voltage, with the dual supply voltages being used to control polarization settings of the LNB, e.g., the lower voltage selecting right hand circular polarization (RHCP) and the higher voltage selecting left hand circular polarization (LHCP). The current drain of the LNB is fairly constant with either of the regulated power supply voltages.
Voltage regulators, which use a controllable series impedance device for maintaining a regulated output voltage coupled to ax load, are susceptible to damage if a short circuit or other fault is applied to the output terminals of the regulator. Such damage often is caused by excessive thermal dissipation of the series impedance device or by exceeding the current rating of the series device. For this reason, it is common to provide overload protection to prevent such damage to the regulator.
One type of overload protection is current limiting in what is known as a "foldback" voltage regulator, such as is disclosed in Easter U.S. Pat. No. 3,445,751. Such a regulator provides output voltage regulation for a changing load until an overload current threshold is reached. For load currents above this threshold, the available output current decreases as the load increases, with a corresponding decrease in the output voltage. The short-circuit current can be adjusted to be but a small fraction of the full load current, thus minimizing the dissipation in the series pass transistor. The voltage regulator of the present invention is such a "foldback" voltage regulator.
Supply current flows from the DC supply source through the emitter-collector path of the series pass transistor to the load. The amount of this current is controlled by a control signal coupled from the output voltage to the base electrode of series pass transistor via an amplification transistor and other circuitry arranged in a negative feedback circuit configuration. In this way, with the voltage drop across the emitter-collector path of the series pass transistor is adjusted to maintain a regulated output voltage.
The series pass transistor incurs a voltage drop under full load, and accordingly dissipates power as part of its regulating function. It is desirable to minimize this power dissipation in the series pass transistor to improve reliability of the series pass transistor, to reduce the cost of the series pass transistor along with associated heat sinks, and to improve the efficiency of the regulation at maximum output voltage by minimizing the voltage difference between the unregulated input voltage and the regulated output voltage.